RENDICONTI DELLA SOCIETA ITALIANA DI MINERALOGIA E PETROLOGIA, 1988, \bI. 4'.2, _pp. 44~·462

Variscan Skarn Ores in South·West Sardinia:their relationships with Cambro·Ordovician stratabound deposits

FABIO AroNTE, GruSEPPINA BALASSONE, MAJuA BONl, LUCA CoSTAMAGNA, GroVANNl DI MAIDDipartimc:nto Scienzc della Terra dell'Universit1l di Napoli, Largo 5. Marcellino 10,80138 Napoli

AI.ISTltACT. - In South-West Sardinia, in addition tothe Lower Paleozoic matabound sedim.entary-diageneticore deposits, some mineralized skarn occurrences exist,associated with pall-tectonic Variscan intrusions ofleucogranites. Their metal content ranges from Fe-Zn­Pb·Ba (already ptesent in the Lower Paleozoicassociations) to F and Cu (with minor As, Mo, WandBi). The metamorphic·metasomatic association consistsof garnets, clinopYroxenes and wollastonite, oftenreplaced by amphiboles, epidotes ard, lastly, chlorites.Thermometamorphic evolution, calculated at pressuresof about 1 Kb, may be subdivided into three stages:metamorphic (T > :;:;O°C), metasomatic {T > 4OQ°Cl,hydrothermal (T < 375°C). The miIll:ralization is linkedto the skarn !ithotypes, whi£h acted as a S1tuetural guideand a trap. We can distinguish in each area betweenmetamorphic (along stratigraphic contacts) and veinskarns (KERRlCK, 1'J77). Most of the ore minerals,consisting of several generations of pyrite, magnetite,pyrrhotite, sphalerite, chalcopyrite and galena, eachshowing about the same paragenetic order of depositionin all the studied areas, are related to the metasomaticand hydrothermal stages.

Ore genesis has been envisaged through aremobilization of metals contained in the LowerPaleozoic stratabound deposits, with a minorcontribution of elements from the intntding magmaticbodies and/or intruded host rocks.

Key words: South·West Sardinia, Variscan, Skaro, Oredeposits.

Introduction

In South-West Sardinia, in addition to thewell-known stratabound ore deposits in theLower Cambrian and Upper Ordovidan

sediments (BoNJ, 1985), other types ofmineralization, smaller in size and thereforeof lesser economic importance, have beenexploited near major intrusive bodies ofVariscan age (Fig. 1). The mineralizationsconsist of high temperatW'e ores, ranging fromskarn to hydrothermal deposits, containingCu, F, As with minor amounts of W, inaddition to Fe, Zn, Pb and Ba which arenormally present in the mineralization ofCambrian and Ordovician age. The type ofmineralization discussed is generally hostedin rocks which were affected by high-gradecontact- metamorphism. Calc-silicate layers,typical of skarn elsewhere in the world, arepresent in these rocks, and they haverecognizable sedimentary and/or diageneticstructures, as fwnd in the older sedimenrs andassociated diagenetic ores. It is interesting tonote that, with only few exceptions, thismineralization can be found in the samesrratigraphic position as the lower Paleozoicstratabound deposits.

On the other hand, the sedimentary fadeshosting the non-metamorphic strataboundores, and those affected by metamorphism,are quite different, so that any meaningfulcomparison between the ores is of necessityincomplete, whereas their affinity can be onlycasual. Furthermore, ther~ are zones, like thatof SE Sulcis, where the mineralization only

446 E APONTE, G. BALASSONE. M. "BQNI, L. COSTAMAGNA. G. DI MAlO

occurs where the host rocks were changed toskam, whereas in the stf'atigraphicallycorresponding horizon no mineralization wasdeveloped. In other llI"fiS. such as theFluminese region, the chief constraints of theores and hosting skarns are tectonic, as wellas the change of permeability betw«nadjacent lithotypes.

The overall genetic framework of the high­temperature ores of SW Sardinia is thereforenot simple, and is open to differentinterpretations. Some authors (e.g. ClAMPI,1909; DRAS, 1957; VIOW & ZueCHINI, 1965)consider the ores of complete epigeneticorigin. Others (e.g. BENZ, 1964; DRAS et al.,1965) have suggested that the skam ores areproducts of metamorphism, which combinesthe elements of the pre-existing mineralizationwith new elements scavenged by hot fluidsfrom the hosting sedimentary rocks, withperhaps an additional contribution from thenearby magmatic intrusives.

In our approach to the problem, weundertook a careful examination of the hostrocks and the mineral paragenesis of the oresin various districts. These studies werecomplemented with sedimentological workand integrated with the acceptedpaleogeographic setting of the LowerPaleozoic terranes. The genetic models werethen discussed in the light of the acquireddata.

Methods of .tudy

The area of study was subdivided intothree districts: Fluminese, Oridda and SESulcis (Fig. 1), each showing differentcharacteristics.

Accurate mapping on a scale of 1:10.000was carried out in the above-mentioneddistricts, in areas underlain by non­metamorphic and high-temperaturemetamorphic rocks. In places, the latter areaslie close to granitic outcrops. 10 othen, thegranitic rocks do not outcrop, and thedistribution of skarns and homfe1s are theonly evidence for an intrusive body at depth.Where possibile, underground mappingwas also carried out, e.g. in the Su Zurfurumine. Stratigraphic sequences, in places

fragmentary, mineralized as well asunminera1ized, were reconstructed, particularattention being focused on thesedimentological characteristics. Thesesequences have been tentatively compared andcarefully sampled. Studies of both polishedand thin sections and X-ray analyses werecarried out. Fluid inclusion studies andmicroprobe analyses of the skam minerals areplanned for future work.

Geological setting of the Lower Paleozoicterrane

Within the autochtonous Cambriansequence of SW Sardinia (COCOZZA, 1979;BECHSTAEDT et al., 1985; 1988) (Fig. 2a) thickcarbonates (GoDnesa Formation) developedon top of carbonate-dastic sediments (NebidaFormation), both being of Lower Cambrianage. The carbonate sequence is overlain bynodular limestones (Campo PisanoFormation), followed by deeper water clastics(Cabitza Formation), comprising the time­span from Middle Cambrian to LowerOrdovician. During the Lower Cambrian SWSardinia was an epicontinental region.Deposition took place in shallow waters,bordered by an erosional area from the E toSE. With time a shallow ramp developed intoa carbonate platform, due to its isolation fromthe source of the clastics. This isolation wascaused by tensional tectonics, resulting in theformation of a basin between the platform andthe (still exposed?) basement (BECHSTAEDT etal., 1988). Tensional tectonics were activefrom late Nebidan times onwards, as shownfor instance by slump structures. Crustalstretching was especially active in Gonnesantimes, as indicated not only by slope sedimentswith their breccias and mass roovements, butalso by ruptural events, which affected theplatform, and its margins.

Numerous stratabound Pb-Zn and Badeposits of Lower Cambrian age are knownand have been partially exploited (BRuSCA &

DESsAu, 1968). These deposits occur withinthe carbonate sequence of the Gonnesa Fm,and locally also within the upper parts of theNebida Fm (Fig. 2b). They were defined byBom (1985) as of Mississippi Valley type.

VARISCAN SKARN ORES IN SOUTH·WEST SARDINIA; THEIR RELATIONSHIPS WITH ETC. 447

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448 F. APONTE, G. BAtASSONE, M. BONI, L. COSTAMAGNA, G DI MAID

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Fig. 2. - Idealized profiles of the Camb-o-Ordovician in SW Sardinia. A) General profile with sedimentary fades;Bl Enlarged profile of the mineralized interval with the location of the stratabound ores; C) Same profile withthe location of the skarn ores: 1) Chia, 2) Mt. Flacca, Is Carillus, S. Luda (Teulada), Fuone Morettu, SideriuBoi, 3) Sa Palma, 4) S. Luda (Fluminimaggiore), :5) Su Zurfuru, 6) Gutturu Pala, Reigraxius, Barraxiuua, Perdu(ana, 7) Arenas p.p., Pcrda Niedda, Tiny, 8) Arenas p.p., 9) Roja Piccioru.

Recently, and a[ leas[ for some of these ores,a complex genedc history is envisaged(BECHSTAEDT &. BONI, in press). Only sparsemineralized occurrences are known in [heuppermost carbonate levels of [he Nebida Fm.

They consist of barhe and layers of galena andsphaleri[e. Locally these deposits areassociated with massive pyrite-marcasite,showing evidence of colloforrn structures anddiagenedc breociation. In Southern Sulds

VARISCAN SKARN ORES IN SOUTH·WE.ST SARDINIA; THEIR REU.TIONSHIPS WITI'! ETC. 449

these mineral occurrences are also associatedwith the upper carbonate lenses, hut arecompletely metamorphosed and also containcopper minerals. Some stratiforrn baritebodies and massive sulfides are foundintercalated in the dolomite sequences of thelower Gonnesa Fm (Dolomia Rigata), whereasthe mineralizations in the upper Gonnesa Fm(Ceroide limestone) are mainly stratabound.

The sulfides in the Ceroide limestoneconsist of sphalerite, pyrite and galena, thelauer sulfide steadily increasing towards thetop of the limestone. All types of orebodiesoccur in different horizons with a variableparagenesis depending on paleogeographicposition (BE.CHSTAEDT and BONt, in press).Two main types of ores are known: yellow«blendoso» limestone, consisting of diffusedimpregnations of light-coloro:l sphalerite inpeloidal mudstone, and breeda ores, withsulfides cementing the breccia components,In Oridda, Fluminese and North Sulcis thelast-mentioned ores were subjected to contact­metamorphism and contain abundant coppersulfides. The Cambrian and LowerOrdovidan sediments and their containedmineralizations were deformed before Up~Ordovician deposition (Sardic phase, STILLE,1939) (Fig. 2a, b). A short-level compressionaffected the area, leading to erosion and thedevelopment of fluviatile conditions andalluvial fans, followed again by shallow marinesequences (LAsKE & BECHSTAEDT, 1987). Thesrratigraphic contact is mostly marked byconglomeratic deposits (the «puddinga»), butalso by silica crust, concretionary iron oxideminerals and a karstic morphology (BONI,1985). Ore deposits, although of lesserimportance than those of Cambrian times, arelinked to the contact between the UpperOrdovician and the deformed and erodedCambrian - Lower Ordovidan sediments (Fig.2b). These deposits consist principally of veinsand paleokarst fillings in the underlyingCambrian carbonates (BENZ, 1964; BoNl,1985). Here iron, barium and lead prevail overzinc. Silver is slightly more abundant than inthe Cambrian ore bodies.

Both Cambrian and Ordovician ores wereaffected by early and late diagenetic processes,strong compressional tectonics (Variscan

phases), magmatism (Variscan granites) anddifferent stages of fluid circulations (BONJ,1985).

Variscan tectonics and metamorphism inSouth-West Sardinia

The Variscm orogeny in Sardinia producedimportant deformations, followed by asignificant late to post-tectonic magmatism(CARMIGNANl et al., 1982). A first distinctionshould be made bc=tween the Iglesiente andNorth - Sulds areas, and Southern Su1cis. Inthe latter area the Variscan deformation andregional metamorphism wete of a higherdegree than in the other districts (MINZONJ,1981).

In Iglesiente and North - Sulcis, the mostcommonly accepted Variscan deformationhistory can be outlined as follows(CARMIGNANI et al., 1982).

First Variscan phase: slight folding witheast-west axes, often superimposed on theSardk phase trends.

Second Variscan phase: main deformationwith north-south folding and very strongschistosity.

Third Variscan phase; slight deformationwith variable fold-axis directions and blockfaulting.

The main structure of the Iglesiente is dueto interference from a system of east-westtrending folds (<<Sardic» or first Variscan?)with the north-south folds of the second mainVariscan phase (CARMIGNANI et al., 1982).

Contrary to the more northern areas of theisland, in South-West Sardinia the limit ofburial diagenesis or anchimetamorphism wasnever exceeded, as stated by PALMERINI andPALMERINI·SITZlA (1978) on the basis of dayminerals content. GHEZZO & RIca (1977), onthe other hand, believe that in Iglesiente­Su1cis, as in South-East Sardinia, ametamorphic grade of lower greenschist facieswas already present. During or after the late­Variscan tectono·metamorphic events, theLower Paleozoic basement was affected by acomplex sequence of successive multipleintrusive events spanning about 20 Ma (Fig.1). The intrusive sequence is, composed asfollows (DJ SIMPuao et al., 1975; BRAUA et

450 F. APONTE, G. BALASSONE, M. BONJ, L. COSTAMAGNA, G. DI MAID

al., 1982; GHEZZO and ORSINI, 1982):

1) Deeply emplaced syn-tectonic and late­tectonic intrusions. No related significantskarn ores are known. They are found mostlyin the central and northern areas of Sardinia.

2) Post-tectonic intrusions, present inSouth-West Sardinia, made up of pinkishbiotite-bearing leucogranites, emplaced atabout 289 ± 1 Ma (DEL MORO et al., 1975).The leucogranites, derived from homogeneoushigh-silica magmas, were produced in anextensional regime (GUASPARRI et al., 1984).The textural and structural features ofleucogranitic intrusions indicate that theentire suite was emplaced at a shallow levelin the crust as a consequence of the lowerwater content of the original magmas. Thecrystallization of leucogranites is thought tohave occurred at pressure of 1 to less than 0,5Kbar, corresponding to depths of 3,8 to lessthan 1,8 Km (GUASPARRl et al., 1984). Thisis in agreement with CONTI et al., (1972), whohypothesized a pressure of 0,5 Kb for the K­feldspar and cordierite hornfels facies in themetamorphic aureole immediately at thecontact of the granite at Domus de Maria, inSouthern Sulcis. However, RETTIGHlERl andTUCCI (1982) suggest a depth of at least 11Km for the emplacement of the granite atCapo Spartivento, which belongs to the earlieremplaced monzo-granitic to gabbro-tonaliticlate-tectonic plutonites. The Ieucogranites, towhich the skarn ores discussed in this paperare mostly related, are usually pervasivelyaltered by several pulses of hydrothermalactivity. Sericite, chlorite and fluorite areoften present. Retrograde boiling has beenhypothesized (GUASPARRI et al., 1984) as wellas sudden changes in temperature and pressuredue also to extensive faulting accompanyingthe intrusions at shallow depths and in rockswith differentiated behaviour. The resultingcontact aureoles in these conditions are ill­defined, and show evidence of telescoping.Leucogranites, and especially theit fine·grained facies and porphyries, are recognizedas the only products of the Variscan batholithdirectly connected with porphyry-type, mostlyMo-bearing mineralizations (GUASPARRl et al.,1984),

South·East Sulcis

In SE Sulcis (area III in Fig. 1), the LowerPaleozoic cerrane is characterized by theprevalence of the clastic sediments of theNebida Fm, with minor outcrops ofcarbonatic rocks of the Gonnesa Fm (LowerCambrian). 1be nodular limestones and slatesof the Campo Pisano and Cabitza Fms arescarce. The change from silico-clasticsedimentation, which occurs on a homoclinalramp, to the carbonates deposited in a rimmedshelf environment, is marked by mixedcarbonatic-siliciclastic sequences, the so-called«Alternanze» (Nebida Fm, Punta MannaMember, BECHSTAEDT et al., 1985). Thecarbonate bodies of the «Alternanze» oftenshow an irregular, lensoid form; the observedfacies range from limestones to dolomites,mostly deposited In shallow waterenvironments. Carbonate lenses belonging tothe Punta Manna Member outcrop in the SESulcis area; especially interesting for the topicdiscussed in the present paper are the lensesof Mt. Flacca, Is Carillus, Sideriu Boi andthose outcropping around Teulada (MorenuVein, S. Lucia, Mt. Calcinaio - GuardiaManna Teulada; Fig. 1). Moreover, othercarbonate lenses, (e.g. those outcropping nearChia and Capo Malfatano) have been relatedto the underlying Matoppa Member by"MINZONI (1981). It should be noted, however,that in SE Sulcis the «basal carbonate», whichin Northern Sulcis and Iglesiente representsthe base of the Punta Manna Mbr, is notknown. The carbonate lenses of the PuntaManna Mbr are locally isolated in the upperclastic sequences or grouped as multiple thinhorizons in between the sandstones (Mt.Flacca, S. Lucia, Morettu Vein, Is Carillus,Sideriu Boil. A different situation can beobserved on the eastern flank of Mt. Calcinaioand at Guardia Manna, where the lenses arenumerous and barren. The carbonate

~ intercalations show many different texturesand facies: some of them have a prevailingalgal-oolitic composition, whereas otherscontain oolites and archaeocyathans, andothers are rich in algal filaments or have aprevailing £laser structure.

Many of these rocks host concentrations ofmixed sulfides and/or magnetite-heamatite, in

VARlSCAN SKARN ORES IN SOUTH.'\lI'EST SARDINIA; THEIR REl.ATJONSHIPS WITH ETC. 451

places of small economic importance(TAMBUllRINI and URAS, 1965; URAS et al.,1965; VIOW and ZUCCHINl, 1965). At thesame time, the carbonate intercalations wereaffected by metasomatism caused by hightemperature fluids circulating in themetamorphic aureoles around the graniticintrusions. The granites outcrop as isolatedapexes and are related to the late and post­!cinematic phases of the Variscan orogeny. Thegranitic rocks produced during the latterphases are essentially leucogranites, to whichthe skam ores are related. The carbonatelenses hosting the mineralization show alsocontact-metamorphism, whereas the contraryis not always trUe. It is important to point outthat the protolith for all the metamorphosedand mineralized lenses consist of carbonateswith «flaser» or wavy bedding structures(Morenu Vein, Is Carillus, S. Luda, SideriuBoL Mt. Flacca).

The differences betw«n the metamorphic­metasomatic parageneses found in the variouslenses may be ascribed to: a) local variationsin the original composition of themetamorphosed rocks; b) variable conditionsof fracturing and of permeability to fluids ofthe host rocks; c) distance from the graniteintrusions (ElNAUDI et al., 1981; VIOW andZUCCHINI, 1965). At any rate, not considuingthe local differences in the minemogicalassociations, we can distinguish betw«n twomain different, often superimposed,parageneses. Of these, one might replace theother, and both should correspond to differentstages of the contact-metamorphicphenomenon. Where still recognizable, theprimary paragenesis consists of: 1) garnet(mainly grossularite with less andraditel +wollastonite + pyroxenes (diopside .hedenbergite) (Sideriu Boil. Wollastonite mayoften be observed only as ghosts inside theandradite crystals, as already described at SaMarchesa by VENERANDI-Pnuu (1981). Thisparagenesis corresponds to high - temperaturemetamorphic processes with minimum temp.540°C (equilibrium tempera~ wollastonite- garnet with Pf:: 1 Kb; Uo-nDA and mAMA,1982).

With decreasing temperature, thisparagenesis is no longer stable and is therefore

replaced almost completely by: 2) amphiboles(mainly tremolite and/or actinolite) + epidote(pistacite and minor clinozoisite) (S. Luda,Mt. Flacca, Morettu Vein).

Finally, the hydrothermal circulation offluids produced a widespread chloritization,in places so strong that it obliterated thepreexisting parageneses (Is Carillus).

The metamorphic phenomenology maytherefore be related to a 3-stage evolution ofthe system: metamorphic stage, metasomaticstage, hydrothermal stage, each of whichdeveloped with variable intensity anddifferent modalities (Fig. 5). At Sideriu Boi,for instance, the prevailing association consistsof grossularite garnet + wollastonite, atMorettu Vem amphiboles and epidotes prevailover garnets, all of them partially replaced bychIorites. At S. Lucia, layers with prevailinggarnet + wollastonite alternate with layerscontaining prevailing epidotes (pistacite andclinozoisite). Lastly, at Mt. Flacca and IsCarillus garnets and wollastonite are oftenreplaced by chlorite.

Most of the metamorphosed carbonatelenses host mixed sulfide and oxidemineralization, consisting of an association ofpyrite, sphalerite, galena, chalcopyrite,pyrrhotite and/or magnetitefhaematite. Localdifferences in the composition and in themetallic associations might be related to thevariable importance of the single stages ofmetamorphic evolution. For instance, agreater enrichment in galena over sphaleriteis noted in those areas where the lasthydrothermal stages were more intense (IsCarillus, Mt. Flacca, Sa Perdaiola). In areasof well-developed higher temperatureparagenesis, for example at Morettu Vein andS. Lucia, the metallic association is noted tobe richer in sulfides and oxydes deposited inthe following sequence: 1) pyrite; 2)magnetite; 3) sphalerite with pyrrhotite andcha1copyrite inclusions; 4) galena andsulphosalts. In the areas where the highesttemperature paragenesis has been lessmodified, the prevailing metallic mineral ishaematite, with subordinate galena, sphaleriteand chalcopyrite (Sideriu Boi; Fig. 3c).

Contact-metamorphic phenomena are notonly limited to the body of the carbonate

452 F. APONTE, G. BALASSQNE, M. &oNt, L COSTAMAGNA, G. DI MAlO

lenses, but they are also found along thestratigraphic contact between the clasticsediments of the Nebicla Fm and thecarbonates (Dolomie RigatefPl.anu Sartu Mbr)of the Gonnesa Fm. An interesting examplemay ~ obsuved at Mt. Sa Palma nearTeulada, where the host rock, consistingof prevailing epidotes and chlorites withminor garnet, contains a small magnetite­haematite orebocly, showing evidence ofmushketovitization and then martitization(RAMOOHR, 1975) (Fig. 3a, b). Minorquantities of galena, sphaJerite and pyrite alsooccur.

The carbonate lenses in the Matoppa Mbrwere also subjected to contact-metamorphism,but in a less obvious way. The layers ofmetamorphic minerals. in the Capo Malfaranolenses are rare. The carbonate lenses, however,are completely marmorized. Metallic mineralsare absent h(U. A similar situation can beobserved in the lens near Chia, where therecrystallization of the carbonate body intoiron gossan, probably originated from theoxydation of stratabound sulfides, has beenobserved.

From the diswssion given alxwe on the oreand gangue minerals of the metamorphicassociation and their geologic setting, it ispossible to consider these metamorphiclithotypes as skarn sensu lato, as defined byBURT (1977), EINAUDI et al., (1981) andEINAUDI and BURT (1982). The structuralsetting and the development processes of theskarn bodies in the areas studied allow afurther classification after KEIooo:: (1977) andEINAUDI and BURT (1982). The situations atMt. Flacca, Is Carillus and Morettu Veinshould be considered typical of vein skarn,emplaced through fault lines, along whichmetamorphic, meusomatic and lastly

(I) In the pre$ent paper the expression «mc:tamorpNcskarn_ and nein skam_, taken by KEuJCK (1977),are in most cases purely descriptive, even if a vari.bleamount of metasomatism is always taken intoaccount.Only locally the diffusion phenomenon is envisagedfor the «metamorphic skarn_, as e.g. in the «£laserlenses_ of S. Luci, and SKleriu Boi in SouthernSukis, where there is evidence of maner exchanaebetween argill.ceous and carbol1litic Iithotypcs, withoccasional COl and Hp contribution.

hydrothermal fluids infiltrated. In places,quartz veins occur along these fault lines (e.g.Morettu Vein). The situation is different atSa Perdaiola and in the small limestone quarryat the junction to the Teulada harbour. Inboth these areas, at the contact betweengranite and carbonate rocks, a metamorphiccomplex, described as a magmatic skarn inthe sense of KERRlCK (1977), was found(metamorphic minerals on the limestoneside. Exoskam, metamorphic minerals on thegranite side .. Endoskarn). The endoskarn isotherwise generally lacking or obliterated bythe subsequent hydrotermal alteration (Cmmet aI., 1970).

Even more different are the metamorphiccomplexes of Mt. Sa Palma and Sideriu Boi.The former, as already mentioned, is at thecontact between the dolomiric limestone ofGonnesa Fm <Dolomite Rigate Mbr) and thesandstone of Nebida Fm (Puma Manna Mbr).Infiltration of hot fluids along thestracigraphic contact, which formed apreferential channelway, might have induceda bi-metasomatic exchange (KERRlCK, 1977)between the constituents of both adjoininglithotypes. Therefore, this process probablyproduced a metamorphic skam, as defined byKERRlCK (1977).

At Sideriu Boi the geologic setting is morecomplex, because here the fluids penetratedalong a fault line, and again at the contactbetween sandstone and carbonate rocks.FoUowing KERRlCK (1977) the larrer shouldbe considered a case between vein- andmetamorphic skarn(1).

Oridd.

In Oridda (area II in Fig. 1) a post-tectonicleucogranitic bOOy (Dr SIMPUOO et al., 1975;GHEZZO and ORsINI, 1982) intrudes the

About the «vein skarn_, the infiltrltiOll mechanismis made dear by the lIbund.n~of fractures thr<lUghwhich the fluids may have travelled and by the veinfilliol! fractures.Real hornfcls, with noo external contributions totheir mincralogy, are rare and, in the wdJ mineralizedareas, difficult to be evidenced, thank to theintenction of the: earlier metamorpbo$ed Iithotypeswith the yoorcer hydrothermall\.lids and conscguentJ'Ctro-metam<ll'phism.

VAR1SCAN SKARN ORES IN SOUTH.WEST SARDINIA; THEIR RELATIONSHIPS WITH ETC. 453

Lower Paleozoic sequences consisting ofCambrian and Ordovician sediments.

The characteristic metamorphic andhydrothermal processes, developed aroundthis body, caused the formation of hornfels,marbles and skarns (DRAS and VIOLO, 1963a,b). In the areas where skarns prevail, the mostcommon Iithotypes are represented by theCeroide limestone (Gonnesa Fm) and silico­clastic Ordovician sediments (<<quartz crusts »,conglomerates and slates). The latter, near thecontact with granites, are widely transformedinto hornfels. In Oridda the contact betweenthe slightly folded Cambrian-LowerOrdovician carbonates and the upperOrdovician clastics is unconformable. Thiscontact is marked by an extended network ofpaleokarS[ic cavities, whi~h acted aschannelways for the metamorphic fluids andcontrolled the deposition of skarn minerals.Another important constraint for skarnformation are the NW-SE and NE-SWtrending major tectonic lines. In thewhole Oridda, however, and also far fromknown intrusions, a thennal low-grademetamorphism, mostly recognizable from thehigher-temperature parageneses of the metallicminerals (e.g. Barrasciutta, Perdu Carta andReigraxius), occurs everywhere. In the above­mentioned mines the ore bodies arestratabound and the host limestones are notrecrystallized. However, among the oreminerals, near the usual sedimentaryassociation (BaNJ, 1985) consisting of pyrite­sphalerite-galena, chalcopyrite (as inclusionsin sphalerite), fluorite and arsenopyrite mayalso be recognized.

The most characteristic skarn outcropsoccur in the areas of Perda Niedda (ClAMPI,1909), Arenas-Tiny (BENZ, 1964) and SaDuchessa (CARTA, 1942; DRAS, 1957), wheresome small mixed sulfide ore bodies have beenexploited. Three main types have beenrecognized: 1) skarn relat~d to stratigraphiccontacts, as e.g. between Gonnesa andCabitza Fms (Sa Duchessa), and alongthe Cambro-Ordovician unconformity,recognizable at Perda Niedda, Arenas-Tinyand Punta Nebidedda (metamorphic skarn,KERRICK, 1977); 2) skarn in small veins in themarmorized Ceroide limestone, along main

tectonic lines, as seen at Sa Duchessa p.p.(vein skarn, KERRICK, 1977); 3) skarn at thecontact between granite and marble, as atPerda Niedda p.p. (magmatic skarn, ICERRICK,1977).

The skarns related to the paleokarsticcavities consist mostly of an association ofandraditic garnet (often sufficiently abundantto form true garnetites, showing ghost ofthe original stratification, as at Tiny,Punta Nebidedda and Perda Niedda p.p.),wollastonite, tremoIitic amphiboles,hedenbergite and minor diopside.

The subsequent lower - temperaturehydrothermal stage is generally very welldeveloped (especially at Perda Niedda), withabundant chlorite (replacing amphiboles,pyroxenes and garnets), sericite, fluorite,calcite and qUart2 (replacing wonastonite). Theparagenetic sequence of the ore minerals atPerda Niedda is as following: pyrite -+

magnetite -+ chalcopyrite -+ sphalerite withchalcopyrite inclusions -+ galena and fluorite.Some inclusions of cobaltite and saffloritehave been observed in the pyrite.Arsenopyrite and marcasite may also occur,as well as small needle-like inclusions ofpyrrhotite in sphalerite (Fig. 4b).

The main ore minerals at Tiny chieflyconsist of galena with less sphalerite andpyrite. These minerals generally fill theinterstices between the metamorphic minerals(e.g. garnets), or may be observed in vein orcavity fillings in the marble. At PuntaNebidedda the general situation is similar asat Tiny. The mineralogic association of gangueminerals consists of andradite garnet,tremolite, epidotes and chlorite.

Both vein and metamorphic skarn havebeen observed at the Sa Duchessa mine, wherethe main ore, exploited in the past, consistedof calamine and copper carbonates enrichedthrough supergene processes in relativelyrecent karstic cavities (CARTA, 1942; DRAS,1957). At Sa Duchessa, the protore consistedof st1'11tabound pyrite-ehalcopyrite bodies withminor sphalerite in the upper zone of amarmorized Ceroide limestone, at the contactwith the Campo Pisano and/or Cabitza Fm(metamorphic skarnl. In the mine area somepoorly developed vein skarns also occur along

454 F. APONTE, G. BALASSQNE, M. 8ONI, L COSTAMAGNA, G. DJ MAlO

VARISCAN SKARN ORES IN SOUTH·WEST SARDINIA; THEIR RELATIONSHIPS WITH En:. 4SS

fracture lines. The small size of the vein skamsis probably responsible foe the lack ofmineralization. The mineralogical associationof the barren vein skarns contains andraditegarnet, tremolite, epidote (pistacite) andchlorites.

The genetic process is essentially the samefor all dU'~ types of skarn mineralization,although they followed variable and diverst:evolutionary paths. At Perda Niedda and SaDuchessa skarn formation was followed byintense hydrothermal alteration, probablyenhanced by the high permeability of the hostrocks through which the fluids circulated. AtTiny and Punta Nebidedda, hydrothermalalteration is much less developed or evenabsent. Owing to this diffetent evolution, therelationships between the metallic and non·metallic associations are also different in thevarious areas. At Perda Niedda and SaDuchessa, the metallic paragenesis is oftenpartly replaced by lower·temperaturehydrothermal minerals, such as sericite,chlorite, calcite and quarz, as indicated byclear reaction rims. At Tiny and PuntaNebidedda, on the other hand, the prevailingore mineral (galena) not only coexists with thehigher-temperature skarn minerals such aswollastonite and andnklite, but does not showany reaction rims. This could be explained bya very early segregation of galena togetherwith the skarn minerals.

From the discussed parageneses, it could beinferred than in Oridda the most likelyinterval for mineralization begins below500°C and ranges betw~n the segregationof the higher· temperature minerals (garnet+ wollastonite + pyroxenes) and thehydrothermal alteration stage (chlorite +epidote + sericite). With decreasingtemperature, the andraditic garnet might have

Fig. J. - a) Magnc:lite crystals p$C\ldomorphs onhaematite (mushketovile), showing abo Iltermartiliution. Sa Palma, pol. sect., N. D, air; hI Zonedmllinetire aystab 00 haematite (mushkelovile). Laterhaemalite can be seen along cryuaJJogr.phic dirtttiOtlli(mani[e~. Sa PaJ.n.., pol. sect., N 11, oil imm; clHaemame bl.ded aystals, partly repJaccd by garnets.Sideriu Boi, pol. sea., N D, oil imm.; d) Sph.alerite wimchalcopyrite indU5iom of dillerenl dimensions. S. I..ucia(Teulada), pol. sect., N. D, oil imm.

reacted releasing iron, which may have formedmagnetite, followed by the deposition ofsulfides, and lastly by the last hydrothermalmineral phases (> 200°C).

Flumincse

The part of the Auminese area studied he~(area I in Fig. 1) mainly contains the lithologicunits belonging to the middle-upperstratigraphic partition of the Loo.ver Paleozoic.In places, these show evidence of intensechermometamorphic and hydrothermalphenomena related to the granite intrusions.In the area investigated no igneous rocksoccur, although locally (e.g. Su Zurfuru) the~is evidence of probable granitic apexes atshallow depths (PRETn and URAS, 1972). Thetectonic grain of the Paleozoic sequences ischaracterized by a series of anticlines andsynclines, with axes running NW-SE,detached in turn by several faults. The oldestoutcropping sequences are represented by thecarbonate lithotypes of the Gonnesa Fm(Lower Cambrian), foUowed sttatigraphicallyby the ncxI.u1ar limestone of the Campo PisanoFm and the clastics of the Cabitza Fm(Middle-Upper Cambrian LowerOrdovician). These Iithotypes areunconformably covered by Upper Ordovicianclastic sequences (microconglomerates andarenaceous slates; LASKE and BECHSTAEDT,1987). The carbonates outcrop along analmost continous line (E-W) from Mt.Gennargentu to Punta Manna de Pubusinuand also as a few isolated blocks (S. Lucia,Punta Pala Su Sciusciu, Giovanni Longu),aligned NW-SE. They mainly consists ofdolomites and limestones with rimmed shelfand shelf margin facies (cherry limestone,slumpings) with frequent breccia horizons. Tothese breccia horizons are related thesedimentary stratabound Pb-Zn ores exploitedat GUtturu Pala (BoNl, 1985). Some smallmineralizations are also related to the inter­Ordovician unconformity. Here, unlike theOridda area, haematite concentrations occurin a small netwCll"k of veins. In the Auminese,the unconformable contact is betw~n twoclastic lithotypes (Cabitza Fm and UpperOrdovician). In this area, therefo~, the

456 F. APONTE, G. MlASSONE, M. BONI, L. COSTAMAGNA, G. DI MAlO

VARISCAN SKARN ORES IN SOunl·WEST SARDINIA; THEIR RELATIONSHIPS WITH ETC. 4.57

mineralized paleokarstic cavities relatedelsewhere to the underlying Camhriancarbonates are ror present. The emplacementof the lare- and post-Variscan leucogranitescaused an intense circulation of high­temperature fluids with formation of tactites(homIds and sl=m) and hydrothennal ve;"'.In some cases the activity of the fluids actedon pre-existing metallic concentrations,resulting in a wide range of a) metamorphosedore bodies; b) skarn mineralization and c)mineralized veins.

The mineralized veins (F-Ba-Pb-Ag) occuronly at S. Lucia (Auminimaggiore; Ruos andVALERA, 1972), whereas at Roja Piccionionly some small skarn veins occur. In theSu Zurfuru - Giovanni Longu area themineralized veins and skarn ores areoverprinted on small pre-exlsting sedimentaryores, also showing evidences of contact­metamorphism (BALASSONE et al., in press).In the Gutturu Pala mine, as in Barrasciutta,Perdu Cacta and Reigraxius (Oridda), higher­temperature effects are slight and have beenobserved only in the ore minerals, as shownby chalcopyrite inclusions (often altered tocovellite) in the sphalerite and rareneoformations of magnetite on pyrite.

The most interesting characters of skamsin the whole Fluminese district, however, areseen in the old 50 Zurfuru mine (F-Ba), owingto a new exploration program which hasallowed the discovery of skarn-related oreoccurrences, rich in Pb-Zn and Cu. The SuZurfuru mine is situated on the northern sideof the Giovanni Longu hill, where the clasticlithotypes belonging to both the Cabitza Fmand the Upper Ordovician predominate. TheGonnesa Fm here consists only of isolatedlimestone blocks with a red matrix-breccia,

Fig. 4. - a) Chakopyritc + pyrrhotilc inclu$iol15 in$phaleritc. Su Zurfuru W, pol. sect., N 11, oli imm; b)Pyrrholitc plalclct$ aligncd alons $phalcrilcC£y$lallographic directio[l$. Perda Nicdda, pol secl., Nn, o.iI imm; cl Oriented wxler-unmixina of mackinawitcin chakopyrilc, both in a $phaleritc matrix. Su ZurfuruW, pol. $«t., N. n, oil imm; d) Pyrrixltitc bcina rcpIacmby intergrowdl$ of pyritc-m:&l'CUilc (upper part of thepbolognph) II'Id pyrite-magnetitc Oower part). SuZurfunJ W, pol. ICCI., N. D, oiJimm;C)PyrrhotiIC bcinarcplac:er;I by intcrgrowlhs of pyrilC-magrEtiIC. Su ZurfuruW, pol. sect., N n. oil imIn.

some of them embedded in the slate and/orOrdovician conglomerate. Some of the blockscontain black chen nodules and slumpedlayers; in places the matrix-breccias haveelongated clasts of white-grey Ceroide.

Owing to strong tectonism in the area andto contact-mrtamorphism, it is difficult todistinguish between Cambriah-LowerOrdovician and Upper Ordovician slates. Thelatter may be recognized with some degree ofceruinty, even if they contain themicroconglomerate fades, also called.:puddingall' (w.sKE and BECHSTAEDT, 1987),at their base. From the tectonic point of view,the Su Zurfuru mine appears to be associatedwith the NW-SE trending Su Zurfuru fault.This is one of 2 imponam structural lineswhich intersect the E-W trending Sa Brocciaanticline (BALASSONE et al., in press).

The metamorphic effects are very differentdepending on the various lithotypes. Thermalmetamorphism of the slates andmicroconglomerates produced hornfe1s, inplaces also showing metasomatic effects,eviden~ by large crystals of garnets, £Iuorite,armenite (BALASSONE et al., in press) andpyrite. Marmorization of the carbonates is themore conspicuous effect, accompanied,especially in the breccia matrix, by neoformedcale-silicates such as clinopyroxenes(hedenbergite), sporadic garnet, amphiboles,quartz and chlorite. Metallic minerals are veryrare. The emplacement of the skarn bodiesfollows some important directions, such asstratigraphic contacts and tectonic directions.Consequently these bodies have generallyelongated shapes, although where bothconditions are accomplished, the skarn bodymay also extend laterally to a considerableextent, reaching massive forms.

The Su Zurfuru mine contains acombination of both metamorphic and veinskarn (KERRlCK, 1977). The former is massiveand occurs along the contact between marbleand metamorphosed slates. 1be latter occursentirely in the marble and is related to faultzones. It may be hypothesized, however, thatboth of them form a single skarn body alongthe marble-slate stratigraphic (?) contact(metamorphic skarnl, from which severalapophyses originate (vein skarnl, probably

"8 F. APONTE, G. lV.LASSONE, M. BONI, L. COSTAMAGNA, G. DJ MAlO

along small joints, acting as channelwaY$ forthe fluids, in the marmorized limestones. Themassive skarn has a mineralogic associationconsisting of andradite gamer, wollaslonire,clinopyroxenes, actinolite and epidotes. Th~early minerals seem to have been replaced atleast partly by lower tem~rature phases, suchas chlorite, generally replacing pyroxenes andamphiboles, quartz and calcite (often showingreaction rims with wollaslonite) and fluodte.The ore minerals in the massive skaen consistof pyrite, sphalerite (with pyrrhotite andchalcopyrite inclusions), galena, pyrrhorite,magnetite, marcasite and haematite. There arealso minerals of supergene origin, such ascovellite and bornite. The most frequentlyobserved parageneses are: pyrite I ­magnetite + pyrrhotite -+ pyrite Il -+­sphalerite (with pyrrhotite and chalcopyriteinclusions) - galena. Some mackinawite sub­exsolutions in the chalcopyrite inclusions havebeen observed. They may be compared withthose described at Sa Marchesa byVENERANDI-PIRRI (1971). According toRAMDOHR (1975), they represent exsolutionproducts from the high-temperaturechalcopyrites. Their occurrence may indicatetemperatures around 250-200°C (Fig. 4c). Inplaces, some pyrite-marcasite intergrowthsreplacing pyrrhotite were found, as well as afew haematite patches replacing magnetite andpyrite I. Most of the ore minerals arecontained in a fibrous network ofwollastonite, pyroxenes and amphiboles.

The so-called vein skarn (or apophyses)consist of sub-rounded bodies with zonedstructure. In fact, the outer rims of thesebodies, in contact with the host marble, showan aureole of idiomorphic elongatedhedenbergite macrocrystals (up to 10 cm inlength) in places largerly replaced by chIorite.These crystals are associated with minorquartz and calcite, and with traces of sulfides(mostly chalcopyrite and galena). The innerpart of the apophyses consist of the samepyroxenes, but the hedenlxrgite crystals aresmaller and with an ipidiomorphic habitus,whereas calcite and quartz are more abundant.The ore concentrations consist of pyrrhotite,chalcopyrite, sphalerite, pyrite, magnetite,marcasite and galena, and are much higher in

the inner zones. 1be mutual relations betweenthe minerals are as follows: pyrite I ­pyrrhotite -sphalerite (with chalcopyrite andpyrrhotite inclusions) -. magnetite +calcsilicate .... pyrite Il -+- chalcopyrite ­galena. In places, it is possibile to observetriple junctions between pyrite I, pyrrhotiteand sphalerite (with the above-mentionedinclusions), which is interpreted as evidenceof the almost synchronous deposition of theseminerals. Pyrite-marcasite (VENERANDI-PrRRl,1971; RAMDOHR, 1975) and pyrite-magnetiteintergrowths, both devdoped after pyrrhotite,are very common (Fig. 4d, e).

Conclusions

From the evidence presented in this work,the following points can be made with regardto the skarn ores of SE Sardinia:

1) The general coincidence (with only afew exceptions) of the stratigraphic positionof the Cambrian and Ordovician strataboundores with the skarn mineralizations (BONI,1985).

2) The additional elements (As, F, W,Cu) in the skams, compared with the LowerPaleozoic sedimentary ores (Fe, Zn, Pb, Ba).

3) The gradual change in ore paragenesesbetween sedimentary-diagenetic low­temperature minerals and high-temperarureassociations, towards the metamorphicaureoles around. the intrusi~ bodies.

4) The association of skarn ores with theupper Nebida Fm in SE SuJcis, the upperGonnesa Fm and the Upper Ordovicianunconformity in Oridda and Auminese (withthe puzzling exception of Su Zurfuru).

5) The association of the skarnmineralized bodies with the post-tectonicleucogranites, already known in literature fortheir small «porphyry copper. potential(GUASPARRJ et al., 1984).

As far as skam genesis is concerned, theseprocesses are mostly controlled by an intensethermal rise and movement of fluids (probablyH20 and CO2) in the areas affected bygranite intrusions. The difftten~s in thelithologic composition of the protoliths, thedifference; in permeability (along fractures orcontacts between diverse lithologies) and the

VARlSCAN SKARN ORES IN SOUTH.WEST SARDINIA; TIiEIR RELATIONSHIPS WITIi ETC. 4.59

metamorphic

I metasomatlcI

( htdtnbngitfI(I ffllD-ladlllolitl

hydrothermalWObs'IlIlIIII --<I (diopSldtI ilQphlbolu \

I I IfIID....j 'lfmohtf chlDfI'U

II !arnlrid,te

sndlSdl1t I gilnf'$ glindlu Is.s.,( PlstiCitf

Iluori'lI 19'GUuli'ilfI fplllOlu <I I I l ChllDZoisi'fI qUltll

..:.: ~. - -.'­uE'ii- • >• • •E ~ •

I

"O••IU - i fILO". "O••TTU i fILO,. • ..O••TTU

.........LIII .. I S" ..... L .... I I" ....L .....

S LUCI ... I III C .... ILLU. I •. LUC''''

PII.O... ,.11100'" : IIIO,.TII fL ...CC ... I DlOII.IU ."."' ....0 ... ,.1.00... I 11 C.... ILLU•I IDU zu.ru.u I TINT I ....0 ... ,.11100"

I DU ZU.fU.U ! DU zu.ru.u

~ '" ....!::!~ ...-~~ ..• 0'

f'LO"1I IOO.ETTU

" C....ILLU.

P••O'" ,.1.00'"

.... OUCH.SS'"

_T... ,..DlOEOO...

DU ZUltfU.U

I

FiB. S. - Diff~m stages of the sUm and ore minerah formation.

variable distances from the granite intrusions,have generated a wide series of metamorphiclithoty~s, including all possible stagesbetween calc·silicatic homfeIs and skam sensustricto.

In SE Sulcis, the sedimentary protolithshave a mixed carbonatic-silty·c1ayeycomposition (__ lenses» of tbe Nebida Fm).Those mostly involved in the formation ofskam minerals are the lenses with flaserstructure, with a low clay content.Metamorphism and mineralization in theupper Nebida Fm in SE Sulcis may havedeveloped with either small or no externalcontribution. The protolith, therefore, evenwhere deeply modified in its mineralogy,might have retained its original chemicalcomposition. The metamorphic lithotypesderived from the laner process should beclassified as calc-siIicate hornfels rather thenreal skarn. In SE Sulcis these situations seemto be common, with only few exceprionsclassificable as metamorphic skarn.

In Oridda and Fluminese, even though inplaces essentially similar situations as in Sulds

occur, the skarn s.s., of both vein andmetamorphic types, are much more frequentin the Gonnesa Fm and at the inter­Ordovician uoconformity. In these skarnevidence of metasomatic exchanges is alsoprovided by the coarser grain size of theminerals (especially garnets and pyroxenes).Moreover, it is of special interest to draw theartemion to the high iron content cif tooeskarns S.S., contained in hedenbergitepyroxenes and andraditic garnets, comparedwith the hornfels of SE Sulcis, whose garnetare predominantly of the grossularitic type.

It is possible, however, summarizing theseveral observed parageneses in the areasstudied, to distinguish 3 main mineralogicassociations (UcHmA and IIYAMA, 1982),which reflect 3 different evolutionary stagesat pressure of about 1 kb (Fig. 5, 6):

1) Metamorphic stage (T > "0°C),characterized by garnet + wollastonite andgamet + pyroxenes (2 generations of garnets,the second being anomalously birefringent andzoned: VERKAEREN and BARTIIOLOME, 1979).

2) Metasomatic stage (T > 400°C),

F. APONTE, G. BALASSONE, M. SONl, L COS'IAMAGNA, G. 01 MAlO

characterized by epidotes and amphiboles(actinolite> ttemolite).

3) Hydrothermal stage (T < 375°C)characterized by chJorite, fluorite, quartz andcalcite.

The metallic associations are similar in all

the studied areas. There are, however, variablepercentages of the elements, which in turnreflect both the content of the originalsuatabound Lower Paleozoic concentrations(BeNl, 1985) and the associatioo characteristicof the metamorphic stages relared to themineralization. To the first, higher·temperature metamorphic stage, only smallpyrite and haematite magnetite(mushketovite) concentrations are related,both coexisting with the garnets (Fig. 5). Tothe metasomatic and hydrothermal stagesmost of the ore minerals are related,comprising several generations of pyrite,magnetite, pyrrhotite, sphalerite, chalcopyriteand galena, each showing about the sameparagenetic order of segregation. The oreminerals appear to have been deposited chieflytogether with the amphiboles and epidotes;they were then partly replaced by chloriteminerals. The temperature for the main phaseof the deposition should therefore rangeIxrween 400 and 200°C (EINAUDJ and BURT,1982), with minor deposition of lower­temperature sulfides coexisting with chlorites(Fig. 5). Critical points for the su1£ides are thecoexistence of chalcopyrite (stable below325°C, SUGAKl et al., 1975) and the presenceof mackinawite exolutions in chalcopyrite(2QO.250·C, RAMooHR, 1975). PADAUNO andVAl.ERA (1977) observed fluid inclusionhomogenization temperature ranging from370°C to lOO°C in the various generationsof fluorite crystals in the Oridda area.

The mineralizing process, however, wasfacilitated by the deposition of the higher­temperature metamorphic minerals such asgarnet, wollastonite and pyroxenes, along thesttatigraphic contacts and structuraldirections. They caused an increase inpermeability in comparison with the formerlithotypes, thus opening the way to major fluidcirculation and at the same time acting as atrap for the metal-bearing solutions.

Finally, to the last hydrothermal phases,with temperature of about lOOoe or less(BoNI, 1985), are rdated the multiplegenerations of vein fillings, containing Ag­bearing galena, fluorite aoo barite, withquartz, calcite and dolomite as gangueminerals.

0.20

<.·0'.

0.150.10OD5

.o"

o

600 P=l kbar

Fig. 6. - T·Xr.... diqram in the system CaO-M&O.AJP,}-Si01-H;O-C01. p_. 1 kb. This ftgW'C ",ascon!lructed on the lISsumplions rhat AI 0, and M&Oare inert components, rhat CaO is a mobiie component,that SiO: is an excess component and that H:O andCO2 are perdectly mobile componenu. Based on thedilllram of UCHlDA and IIYMA (1982). 1) metamorphicslllle associations; 2) melasomadc SUIte associations.Abbreviations: An. anortite, Tr. tremolile,Di. diopside, Gr. grolSu!arile, Zo. zoisile,Wol. wolla!lornte, Ca!. calcite, Q. quarlz. Theapplicarion of this dilliram to our pangeneses is onlyapproximate.

VARISCAN SKARN ORES IN SOUTH.WEST SARDINIA; THEIR RELATIONSHIPS WIlH ETC. 461

It is reasonably certain that, at least forsome of the investigated areas, the origin ofthe metals in the skarn mineralization is fromthe Cambro-Ordovician stratabound ores.Indeed, in places it is possible to observe agradual transition between non-metamorphicand metamorphic paragenesis. The firstindication of a rise in temperatute is theappearance of cha1copyrite inclusionskhaIcopyrite «disease», cf. CRAIG andVAUGHAN, 1981) in sphalerite, followed by thereplacement of pyrite by magnetite. However,there is no conclusive explanation for thesource of Cu in the skarns, since this metalis absent from the Lower Paleozoicmineralizations of SW Sardinia.

Whether large-scale remobiIization of leadduring the late Variscan period ofmineralization really occurred remains an openquestion in view of the lead data fromfeldspars of the leucogranites (BONI andKOEPPEL, 1985). They are isotopically quitesimilar to the lead of galenas from bothcontact-metamorphosed and some of the veinfilling ores. 1be feldspar lead thus raises somedoubts that the late Variscan ores owe theirexistence only to remobilization of largeamounts of Lower Paleozoic ores. It thereforeseems possible that at least some of thesedeposits, in spite of their «stratabound»appearance, are of more complex origin.

o

Acknowledgements. - We would like to thank the Soc.SIM for granting access to the Su Zurfuru and GutturuPala mines and all staff for their assistance and helpfulsuggestions, 1he present work was funded by the projectMPl 40% 1985-86 (M. Boni). We are also grateful toProEs. M.T. Einaudi, F. Pirajno, G. Tanelli and 1.Venerandi for their stimulating discussions.

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462 F. APONTE, G. BALASSONE, M. BONI, L. COSTAMAGNA, G. DI MAlO

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